A torque converter is a fluid coupling that is used to transfer rotating power from a prime mover, such as an engine or electric motor, to a power-transferring device such as a transmission. The transmission is an apparatus configured to transmit power and torque from a vehicle's prime mover to a load-bearing device such as a drive axis. Conventional transmissions include a variety of gears, shafts, and clutches that transmit torque through the transmission at finite, stepped gear ratios.
Referring to FIG. 1, a conventional powered vehicle system 100 is shown with an internal combustion engine 102 (the prime mover), for example, configured to rotatably drive an output shaft 108 that is coupled to a pump shaft (not shown) of a conventional torque converter 106. The torque converter 106 provides a power input to a conventional transmission 104. The pump shaft (not shown) of the conventional torque converter 106 is coupled to an impeller or pump (not shown) that is rotatably driven by the output shaft 108 of the engine 102. The conventional torque converter 106 can further include a turbine (not shown) that is coupled to a turbine shaft 110. The turbine shaft 110 is coupled to, or integral with, a rotatable input shaft 114 of the conventional transmission 104. The conventional transmission 104 can include a planetary gear system 112 having a plurality of automatically selected gears. An output shaft 116 of the transmission 104 can be coupled to (or integral with) and rotatably drive a propeller shaft (not shown) that is coupled to a conventional universal joint of a vehicle via an output yoke 118 or the like. The conventional torque converter 106 can further include a lockup clutch assembly (not shown) that is located within the torque converter. The lockup clutch assembly directly connects the engine 102 to the input shaft 114 of the transmission 104. At cruising speed, the engine and the transmission are directly coupled together, which in turn increases fuel economy.
Conventional torque converters can operate in different modes, such as “converter mode” and “lockup mode”. The operation of the torque converter in “converter mode” can occur, for example, during vehicle launch, low speed operation and certain shifting conditions. In “converter mode”, the lockup clutch is disengaged and the pump rotates at the rotational speed of the engine output shaft while the turbine is rotatably actuated by the pump through a fluid interposed between the pump and turbine.
The operation of the conventional torque converter in “lockup mode” can occur, for example, when certain gears of the planetary gear system of the transmission are engaged. In this mode, the lockup clutch is engaged and the pump is coupled to the turbine so that the engine output shaft is directly coupled to the input shaft of the transmission.
There are different types of torque converters. Two of the different types of converters include a three-pass torque converter and a two-pass torque converter. The three-pass torque converter has an independent lockup piston cavity that is separate from a converter cavity inside the torque converter. As such, flow inside the torque converter through each cavity is independent. In particular, the independent lockup cavity provides the three-pass torque converter to have positive oil feed volume for lockup clutch apply. This is advantageous for three-pass torque converters because the lockup clutch apply procedure can be easily controlled regardless of a vehicle's driving condition.
A conventional two-pass torque converter has fewer components and is therefore less expensive to manufacture than a conventional three-pass torque converter. However, unlike the three-pass torque converter, the conventional two-pass torque converter does not have an independent lockup piston cavity that is separate from a converter cavity. Instead, the direction of oil flow in the two-pass torque converter is reversed when shifting between converter mode and lockup mode. This reversal of oil flow, however, can cause variability in the lockup apply pressure, particularly as the temperature of the oil varies and the vehicle operation changes.
When the transmission is placed in neutral with the engine running, the output of the transmission is disconnected from the wheels to insure that the wheels do not move. When the vehicle is braked, for instance at a stop light, the engine speed is at an idle speed which is insufficient to overcome the application of the brakes. In this condition, the output of the transmission remains connected to the wheels. In each of these situations, however, the turbine within the torque converter continues to spin since the pump continues to direct oil toward the turbine, albeit at much slower rate than when the vehicle is moving.
Even though the lockup piston is disengaged to insure that the turbine continues to spin freely, a certain amount of oil is located between the lockup piston and the torque converter cavity. The presence of this oil, under some conditions, is sufficient to generate friction between the torque converter cover and the piston. If this amount of friction is large enough, a sufficient amount of heat is generated which can wear and warp the components of the torque converter.
Consequently, there is a need to reduce the friction which can occur between the lockup piston and the cover, thereby reducing the wear and warping of the plates.